The study investigates the individual developmental course of 80 children, two children each in 40 families, in the context of the development of their relationship with their mother. The sample was selected from a larger ongoing study of children of affectively ill parents (MH-02144): 20 families with parental unipolar or bipolar affective illness and 20 families with no parental psychiatric diagnosis. Between the two children in each family, the study covers the developmental course from early to late childhood, or early adolescence. The data source of approximately 20 hours videotaped observations consists of three half-day sessions at Time 1, when the children's ages are 2 to 3 1/2 years for the younger and 5 to 8 years for the older sibling, and two half-day sessions at Time 2, when the children's ages are 5 to 6 1/2 for the younger and 8 to 11 years for the older sibling.